1. Field of the Invention
The present invention relates to pre-equalization in a mobile communication system employing an Orthogonal Frequency Division Multiplexing (hereinafter referred to as “OFDM”) scheme and a Time Division Duplex (hereinafter referred to as “TDD”) scheme, that is, an OFDM/TDD mobile communication system, and more particularly to an adaptive channel prediction apparatus and an adaptive channel prediction method for changing channel prediction depending on the degree of downlink channel variation in uplink pre-equalization using an estimated downlink channel.
2. Description of the Related Art
Next-generation multimedia systems require the transmission of high-capacity data at high speeds in a mobile wireless channel environment and must meet a rapidly increasing demand for wireless communications. Accordingly, research is actively being conducted in order to provide a transmission scheme which has high frequency utilization efficiency while enabling high-speed data transmission. In order to transmit data having a short symbol duration at a high speed, it is necessary to ensure a wide transmission band and to prevent frequency-selective fading from being caused by Inter-Symbol Interference (ISI) or Multi-Path Interference (MPI) of wireless channels. To this end, a multi-carrier technique has been developed, in which a symbol duration is extended and a data symbol is transmitted over a plurality of carriers.
An OFDM scheme, one of the multi-carrier techniques, is advantageous in that frequency utilization efficiency is high due to data transmission over mutually orthogonal sub-carriers whose frequency bands overlap, and fast implementation of modulation/demodulation using a multi-carrier is possible through IFFT (Inverse Fast Fourier Transform) and FFT (Fast Fourier Transform). Further, in the OFDM scheme, it is possible to achieve equalization with one tap per sub-carrier by using a guard interval longer than the delay spread of a signal, resulting from a multi-path channel, for maintaining orthogonality between sub-carriers.
Pre-equalization is a method for compensating for channel distortion prior to transmission by using the equalization simplicity of an OFDM system and reciprocity of a TDD channel. As used herein, the reciprocity of a TDD channel refers to the property that correlation between uplink and downlink channels is very high because both the uplink and downlink channels use the same frequency, and characteristics as well as Doppler frequencies of both the channels are the same in the case of simultaneous transmissions over both channels. Therefore, a downlink channel is estimated in a TDD mode system in order to use it for reception equalization, and the estimated downlink channel can also be used for compensating for the distortion of an uplink channel prior to transmission. The equalization at a receiving-end generally results in noise amplification and thus performance deterioration, but the re-equalization has an advantage in that there is no noise amplification because it is performed prior to transmission. Further, since the equalization at a receiving-end is not needed when the pre-equalization is performed, the pre-equalization is characterized in that it can reduce the complexity of a receiver.
In general, the pre-equalization may be largely divided into two schemes. One scheme is for a case where the reciprocity of a TDD channel is maintained, and the other scheme is for a case where the reciprocity of a TDD channel is not maintained because of the Doppler effect according to terminal movement.
In the pre-equalization scheme for the case where the reciprocity of a TDD channel is maintained, that is, the case where there is almost no terminal movement or a slot interval is sufficiently smaller than a coherence time of the channel, the pre-equalization is performed in such a manner that a channel coefficient is estimated in the last symbol interval of a downlink period in a terminal, and the estimated channel coefficient is used for equalizing all symbols in a next uplink period.
Reference will now be made to FIG. 1, a structural block diagram illustrating an OFDM transceiver in a terminal, which is operative to perform pre-equalization in a case where the reciprocity of a TDD channel is maintained.
A signal transmitted from a base station goes through a multi-path channel and is received with noise added thereto through an antenna of the terminal. The signal received through the antenna passes through a duplexer 100 and is input into a RF processor 102, which in turn down-converts the input signal into an IF (Intermediate Frequency) band signal. Subsequently, an A/D (Analogue-to-Digital) converter (not illustrated) converts the analogue IF band signal into a digital base band signal, and then a burst symbol extractor (not illustrated) extracts an OFDM symbol from the digital base band signal. The OFDM symbol extracted by the burst symbol extractor is processed in such a manner that its CP (Cyclic Prefix), which a transmitting side has inserted therein, is removed by means of a CP remover 104, is subjected to fast Fourier transform through a FFT (Fast Fourier Transformer) 106, and then is applied to a reception equalizer 108. The reception equalizer 108 compensates for channel distortion for the fast Fourier transformed data signal according to a channel characteristic value estimated by means of a channel estimator 112. This channel distortion-compensated signal is demodulated in a demodulator 110.
A transmission equalizer 114 performs the pre-equalization in such a manner that a channel coefficient estimated in the channel estimator 112, that is, a channel coefficient estimated in the last symbol interval of a downlink period, is used for equalizing all symbols in a next uplink period. A coherence time represents a statistic of time intervals over which the impulse response of a channel hardly varies. Combining algorithms such as MRC (Maximal Ratio Combining), EGC (Equal Gain Combining), ZF (Zero Forcing) and MMSE (Minimum Mean Square Error) algorithms may be used for the equalization at a receiving-end, which is performed to provide for the pre-equalization. Further, it is possible to utilize SINR (Signal to Interference plus Noise Ratio)-based pre-equalization methods in which transmission power is restricted in the same manner as in the case of not using the pre-equalization.
Symbols output from the transmission equalizer 114 are transmitted to the base station via an IFFT (Inverse Fast Fourier Transformer) 116, a CP inserter 118, a RF processor 120, and the duplexer 100.
In the case where the reciprocity of a TDD channel is not maintained because of the movement of a terminal or other channel environments, it is necessary to perform a procedure of predicting uplink channel variation based on downlink channel variation by using a downlink channel value estimated prior to pre-equalization because an estimated downlink channel is not the same as an uplink transmission channel. That is, when the reciprocity of a TDD channel is not maintained as stated above, characteristics of the uplink channel change from those of the downlink channel, and thus serious deterioration of pre-equalization performance is caused by incomplete equalization. Thus, the uplink channel variation needs to be predicted such that performance deterioration is prevented from being caused by the incomplete equalization.
Reference will now be made to FIG. 2, which is a structural block diagram illustrating an OFDM transceiver in a terminal, which is operative to perform channel prediction in a case where the reciprocity of a TDD channel is not maintained. Except for a channel estimator 212 and an uplink channel predictor 213, the OFDM transceiver in FIG. 2 has the same internal structure as that of the OFDM transceiver in FIG. 1, and thus a description of the remaining parts will be omitted.
A received signal is fast Fourier transformed by means of the FFT 106, and then is applied to the reception equalizer 108. The reception equalizer 108 compensates for channel distortion for the fast Fourier transformed data signal according to a channel characteristic value estimated by means of the channel estimator 212. This channel distortion-compensated signal is demodulated in a demodulator 110.
The uplink channel predictor 213 performs uplink channel prediction by using the channel coefficient estimated in the channel estimator 212. The uplink channel predictor 213 may use a channel prediction method in which channel variation is detected through interpolation or curve fitting of an estimated channel in a time domain, and extrapolation is applied based on the detected channel variation. There is another method for calculating a statistically predicted value from an estimated downlink channel value by using a Bessel function. The channel estimated in a time domain is converted into a channel in a frequency domain so as to perform pre-equalization in the transmission equalizer 114.
As stated above, each of the two conventional pre-equalization schemes employs an OFDM transceiver of a terminal, which is independently applied in the case where the reciprocity of a TDD channel is maintained or in the case where the reciprocity of a TDD channel is not maintained. However, it is common that a terminal is subject to both cases, that is, a case where the terminal is moved and a case where the terminal is stationary. Thus, if a terminal uses an OFDM transceiver taking only one case into consideration, only channel estimation or channel prediction optimized to the corresponding case can be performed. For example, when a terminal is moving, a channel prediction method is always applied in order to perform pre-equalization for the case where the reciprocity of a TDD channel is not maintained, there is a disadvantage in that unnecessary channel prediction is performed even in a situation where the reciprocity of a TDD channel is maintained. Further, although the degree of the reciprocity of a TDD channel varies with a terminal moving speed and an optimal channel prediction method also changes accordingly, it is impossible to control the channel prediction method according to respective situations.